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<div class="chapter">
<div class="titlepage"><div><div><h2 class="title">
<a name="howto-interface"></a>How to define and implement interfaces</h2></div></div></div>
<div class="toc"><dl>
<dt><span class="sect1"><a href="howto-interface.html#howto-interface-define">How to define interfaces</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-implement.html">How To define implement an Interface?</a></span></dt>
<dt><span class="sect1"><a href="ch06s03.html">Interface definition prerequisites</a></span></dt>
<dt><span class="sect1"><a href="howto-interface-properties.html">Interface Properties</a></span></dt>
</dl></div>
<div class="sect1">
<div class="titlepage"><div><div><h2 class="title" style="clear: both">
<a name="howto-interface-define"></a>How to define interfaces</h2></div></div></div>
<p>
    The bulk of interface definition has already been shown in <a class="xref" href="gtype-non-instantiable-classed.html" title="Non-instantiable classed types: interfaces">the section called “Non-instantiable classed types: interfaces”</a>
    but I feel it is needed to show exactly how to create an interface.
  </p>
<p>
    As above, the first step is to get the header right:
</p>
<pre class="programlisting">
#ifndef __MAMAN_IBAZ_H__
#define __MAMAN_IBAZ_H__

#include &lt;glib-object.h&gt;

#define MAMAN_TYPE_IBAZ                 (maman_ibaz_get_type ())
#define MAMAN_IBAZ(obj)                 (G_TYPE_CHECK_INSTANCE_CAST ((obj), MAMAN_TYPE_IBAZ, MamanIbaz))
#define MAMAN_IS_IBAZ(obj)              (G_TYPE_CHECK_INSTANCE_TYPE ((obj), MAMAN_TYPE_IBAZ))
#define MAMAN_IBAZ_GET_INTERFACE(inst)  (G_TYPE_INSTANCE_GET_INTERFACE ((inst), MAMAN_TYPE_IBAZ, MamanIbazInterface))


typedef struct _MamanIbaz               MamanIbaz; /* dummy object */
typedef struct _MamanIbazInterface      MamanIbazInterface;

struct _MamanIbazInterface
{
  GTypeInterface parent_iface;

  void (*do_action) (MamanIbaz *self);
};

GType maman_ibaz_get_type (void);

void maman_ibaz_do_action (MamanIbaz *self);

#endif /* __MAMAN_IBAZ_H__ */
</pre>
<p>
    This code is the same as the code for a normal <a class="link" href="gobject-Type-Information.html#GType" title="GType"><span class="type">GType</span></a>
    which derives from a <a class="link" href="gobject-The-Base-Object-Type.html#GObject"><span class="type">GObject</span></a> except for a few details:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p>
        The <code class="function">_GET_CLASS</code> macro is called <code class="function">_GET_INTERFACE</code>
                  and not implemented with <code class="function"><a class="link" href="gobject-Type-Information.html#G-TYPE-INSTANCE-GET-CLASS:CAPS" title="G_TYPE_INSTANCE_GET_CLASS()">G_TYPE_INSTANCE_GET_CLASS</a></code>
                  but with <code class="function"><a class="link" href="gobject-Type-Information.html#G-TYPE-INSTANCE-GET-INTERFACE:CAPS" title="G_TYPE_INSTANCE_GET_INTERFACE()">G_TYPE_INSTANCE_GET_INTERFACE</a></code>.
      </p></li>
<li class="listitem"><p>
        The instance type, <span class="type">MamanIbaz</span> is not fully defined: it is
        used merely as an abstract type which represents an instance of
        whatever object which implements the interface.
      </p></li>
<li class="listitem"><p>
        The parent of the <span class="type">MamanIbazInterface</span> is not
        <span class="type">GObjectClass</span> but <span class="type">GTypeInterface</span>.
      </p></li>
</ul></div>
<p>
  </p>
<p>
    The implementation of the <span class="type">MamanIbaz</span> type itself is trivial:
    </p>
<div class="itemizedlist"><ul class="itemizedlist" type="disc">
<li class="listitem"><p><code class="function">maman_ibaz_get_type</code> registers the
       type in the type system.
       </p></li>
<li class="listitem"><p><code class="function">maman_ibaz_base_init</code> is expected 
      to register the interface's signals if there are any (we will see a bit
      (later how to use them). Make sure to use a static local boolean variable
      to make sure not to run the initialization code twice (as described in
      <a class="xref" href="gtype-non-instantiable-classed.html#gtype-non-instantiable-classed-init" title="Interface Initialization">the section called “Interface Initialization”</a>, 
      <code class="function">base_init</code> is run once for each interface implementation 
      instantiation)</p></li>
<li class="listitem"><p><code class="function">maman_ibaz_do_action</code> dereferences
      the class structure to access its associated class function and calls it.
      </p></li>
</ul></div>
<p>
</p>
<pre class="programlisting">
static void
maman_ibaz_base_init (gpointer g_class)
{
  static gboolean is_initialized = FALSE;

  if (!is_initialized)
    {
      /* add properties and signals to the interface here */

      is_initialized = TRUE;
    }
}

GType
maman_ibaz_get_type (void)
{
  static GType iface_type = 0;
  if (iface_type == 0)
    {
      static const GTypeInfo info = {
        sizeof (MamanIbazInterface),
        maman_ibaz_base_init,   /* base_init */
        NULL,   /* base_finalize */
      };

      iface_type = g_type_register_static (G_TYPE_INTERFACE, "MamanIbaz",
                                           &amp;info, 0);
    }

  return iface_type;
}

void
maman_ibaz_do_action (MamanIbaz *self)
{
  g_return_if_fail (MAMAN_IS_IBAZ (self));

  MAMAN_IBAZ_GET_INTERFACE (self)-&gt;do_action (self);
}
</pre>
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